a. Field of Invention
The invention relates generally to a method and apparatus for plastic casting, and, more particularly to a method and apparatus for powder slush plastic casting using a multi element moving vacuum chamber, surrounding a casting tool and a powder box, for reducing imperfections such as pin holes, surface voiding and skin thickness variations in a finished cast product.
b. Description of Related Art
A need exists for an improved method and apparatus for powder slush plastic casting, for example in skinned automotive instrument panel manufacturing, whereby common defects such as pin holes, surface voiding and skin thickness variations may be reduced.
Powder slush casting (or molding) of thin-walled articles from castable plastic material, such as plastisol and pourable plastic powders, is a well known method of manufacturing molded plastic articles, such as hollow articles, automotive dashboards, door panels and the like. Powder slush casting is particularly useful in the manufacture of hollow articles since the wall thickness of the article can be controlled by adjusting the amount of plastisol or plastic powder used in the casting process. Examples of commonly used plastic powders include Thermo Plastic Urethane (T.P.U.), Thermo Plastic Olefinic (T.P.O.) and Poly Vinyl Chloride (P.V.C.).
In conventional slush casting, a mold is first preheated. FIGS. 1A-1F are exemplary diagrams of a related art powder casting cycle illustrating the key stages of a conventional casting cycle. As shown in FIGS. 1A and 1B, a powder box 1, filled with plastic powder 2 is then brought into contact with pre-heated mold 3 and engaged with it to prevent leakage of plastic powder 2. As next shown in FIG. 1C, when mold 3 is rotated, plastic powder 2 strikes the heated mold surface 4. When mold 3 stops rotating, powder 2 on mold surface 4 fuses to form the shape of mold surface 4 (see FIG. 1D), and any remaining plastic powder 2 drains back into powder box 1 for subsequent casting, or is discarded. As shown in FIG. 1E, powder box 1 is then disengaged from mold 3 and returned to its original location (shown also in FIG. 1A). Finally, as shown in FIG. 1F, mold 3 is rotated to allow an operator to remove the fused layer (or skin) on mold surface 4.
In the conventional slush casting method and apparatus discussed above, pin holes, surface voiding and skin thickness variations are common defects. In automotive panels for example, pin holes cause urethane foam leakage, and skin thickness variations cause subtle distortions in the finished part""s surface. From a quality stand-point, these defects detract from the product""s appearance, and from a manufacturing stand-point, these defects are the cause of significant scrap levels.
In the art, there currently exist various methods and apparatus for slush casting, as disclosed for example in U.S. Pat. Nos. 6,099,771, 6,082,989, 6,019,590, 5,932,162, 5,840,236, 5,580,501, 5,397,409, 5,387,390, 5,290,499, 5,221,539, 4,946,638, 4,898,697, 4,790,510, 4,740,337, and 4,714,424. The slush casting methods and apparatus disclosed in these patents share the disadvantages of pin holes, surface voiding and skin thickness variations in the finished cast product, as discussed above.
The invention solves the problems and overcomes the drawbacks and disadvantages of the prior art by providing a method and apparatus for powder slush plastic casting, which minimizes defects such as pin holes and evens plastic flow to reduce thickness variations in a finished product, and which is relatively simple and inexpensive to install and operate.
In particular, the invention accomplishes this by providing three embodiments of a multi element moving vacuum chamber (MEMVC) for rotational casting. The first embodiment of the MEMVC includes a mold having a predefined shape and an axis of rotation. The mold is rotatable about the axis of rotation, and includes an inner mold surface and an outer mold surface. The MEMVC further includes a box containing casting material. The box has an inner box surface for holding the casting material and an outer box surface. A first vacuum element is mounted to the outer mold surface and a second vacuum element is mounted to the outer box surface. The mold and the first vacuum element are detachably engageable with the box and the second vacuum element to define an engaged configuration. When the mold, the box and the first and second vacuum elements are in the engaged configuration, air within an enclosure defined by the outer mold surface and an inner surface of the first vacuum element, by the inner mold surface and the inner box surface, and by the outer box surface and an inner surface of the second vacuum element, may be removed to create a vacuum.
In the first embodiment of the MEMVC discussed above (and for the second and third embodiments discussed below), the mold may made of Nickel or another metal. The casting material may be Thermo Plastic Urethane (T.P.U.), Thermo Plastic Olefinic (T.P.O.), Poly Vinyl Chloride (P.V.C.), or a castable plastic. The first and second vacuum elements may be bell shaped, and may each include a vacuum connection for removal of air from an enclosed area defined by the inner surface of the first vacuum element and the outer mold surface, by the inner surface of the second vacuum element and the outer box surface, and further defined by the inner mold surface and the inner box surface. The mold, the first vacuum element, the box and the second vacuum element may further include a vacuum rated seal for providing sealed engagement between each of the mold and the first vacuum element, and the box and the second vacuum element. The mold, the box, the first vacuum element and the second vacuum element may further include a latch (on each) for permitting detachable engagement between the mold and the first vacuum element, and the box and the second vacuum element.
The second embodiment of the MEMVC for rotational casting includes a mold having a predefined shape and an axis of rotation. The mold is rotatable about the axis of rotation and further includes an inner mold surface and an outer mold surface. The MEMVC further includes a box containing casting material. The box has an inner box surface for holding the casting material and an outer box surface. The MEMVC yet further includes a vacuum element mounted to either the outer mold surface or the outer box surface, thereby defining a vacuum element mounted component and a non-vacuum element mounted component. Specifically, if a vacuum element is mounted on the mold, the vacuum element mounted component would be the mold with the vacuum element, and if a vacuum element is mounted to the box, then the vacuum element mounted component would be the box with the vacuum element. The non-vacuum element mounted component is capable of vacuum loading. The mold is detachably engageable with the box, and the vacuum element is detachably engageable with the non-vacuum element mounted component, to define an engaged configuration. When the mold, the box, and the vacuum element are in the engaged configuration, air within an enclosure defined by the inner mold surface and the inner box surface, and further defined by an outer surface of the vacuum element mounted component and an inner surface of the vacuum element, may be removed to create a vacuum.
The third embodiment of the MEMVC for rotational casting includes a mold having a predefined shape and an axis of rotation. The mold is rotatable about the axis of rotation and has an inner mold surface and an outer mold surface. The mold is also capable of vacuum loading. The MEMVC also includes a box containing casting material. The box has an inner box surface for holding the casting material and an outer box surface, and is also capable of vacuum loading. The mold is detachably engageable with the box to define an engaged configuration. When the mold and the box are in the engaged configuration, air within an enclosure defined by the inner mold surface and the inner box surface may be removed to create a vacuum.
In yet another aspect of the invention, the invention solves the problems and overcomes the drawbacks and disadvantages of the prior art by providing a first method for rotatable casting. The method includes providing a mold having a predefined shape and an axis of rotation. The mold has an inner mold surface and an outer mold surface. The method further includes providing a box containing casting material and having an inner box surface for holding the casting material and an outer box surface. The box is located relative to the mold at a first location. The method then includes the steps of mounting a first vacuum element to the outer mold surface and mounting a second vacuum element to the outer box surface. Next, the method includes the steps of heating the mold to a predetermined temperature and moving the mold relative to the box to engage the mold with the box and the first vacuum element with the second vacuum element, thereby defining an engaged configuration. In the engaged configuration, the method includes the step of removing air within an enclosure defined by an inner surface of the first vacuum element and the outer mold surface, by an inner surface of the second vacuum element and the outer box surface, and by the inner mold surface and the inner box surface, in order to create a vacuum in the enclosure.
The invention also provides a second method for rotatable casting, which includes providing a mold having a predefined shape and an axis of rotation. The mold has an inner mold surface and an outer mold surface. The method further includes providing a box containing casting material and having an inner box surface for holding the casting material and an outer box surface. The box is located relative to the mold at a first location. The method then includes the step of mounting at least one vacuum element to one of the outer mold surface or the outer box surface, thereby defining a vacuum element mounted component and a non-vacuum element mounted component. The non-vacuum element mounted component is capable of vacuum loading. The method yet further includes the steps of heating the mold to a predetermined temperature, and moving the mold relative to the box to engage the mold with the box, thereby defining an engaged configuration. In the engaged configuration, the method includes the step of removing air within an enclosure defined by an inner surface of the vacuum element and an outer surface of the vacuum element mounted component, and by the inner mold surface and the inner box surface, in order to create a vacuum in the enclosure.
The invention also provides a third method for rotatable casting, which includes providing a mold having a predefined shape and an axis of rotation. The mold has an inner mold surface and an outer mold surface, and is capable of vacuum loading. The method further includes providing a box containing casting material. The box has an inner box surface for holding the casting material and an outer box surface. The box is located relative to the mold at a first location and is capable of vacuum loading. The method then includes the steps of heating the mold to a predetermined temperature and moving the mold relative to the box to engage the mold with the box, thereby defining an engaged configuration. In the engaged configuration, the method includes the step of removing air within an enclosure defined by the inner mold surface and the inner box surface.
It should be noted that the first through third methods for rotatable casting, discussed above, include all the features of the first through third embodiments of the MEMVC, discussed above.